Doping of silicon crystal slices is old in the art and many types of furnaces have been used to diffuse the dopant material into the crystal surface. Generally, the slices were first coated with a solution or mixture containing the dopant material and then, the slices were spread on a tray and placed in a furnace where they were subjected to a heating, cooling cycle involving many hours at a high temperature. Such a procedure is subject to several problems, such as; (1) warpage of slices due to change in thermal conditions, (2) autodoping between slices, and (3) the proper maintenance of surface concentration of dopant material. Also, only a few slices can be accommodated on each furnace tray, thereby severely limiting the number of slices per furnace cycle.
The present invention avoids many of the disadvantges and limitations of prior art methods. A tubular furnace is used and its central temperature can be kept at the maximum doping value. All the slices are stacked, surface-to-surface in a quartz boat after the dopant material has been added, and then the boat is slowly pushed into the high temperature zone where the diffusion takes place. The stack of slices is clamped together to eliminate both warpage and autodoping between slices. The furnace is small compared to prior art furnaces and several hundred slices can be accommodated in a single small boat.